TECHNOLOGY LED DROOP
distribution of holes among the wells with the total number of holes injected. At the interface between the p-type GaN and the undoped GaN barrier, there can be a reservoir for holes that fills up, prior to injection into the active region (see Figure 4). If the LEDs have a higher barrier, hole injection efficiency may be lower, but once holes overcome the potential barrier, they will gain energy. This increase in energy when entering the active region will influence the capturing efficiency of holes in each of the wells, which have different energies.
Figure 4.
Electroluminescence (EL) spectra of triple- wavelength-LEDs with p-In0.015 p-In0.035
Ga0.985 Ga0.965
N and N layers. The
lower injection efficiency for the LEDs with a higher hole barrier diminishes with increasing current. Injected holes overcoming the higher potential barrier can then be transported farther, resulting in more uniform hole distributions among the MQWs
the distribution of these carriers alters the electronic band structures and radiative and Auger recombination rates.
It is not that challenging to realise a uniform distribution of electrons among multiple quantum wells, but when it comes to holes, this is very tough. In a conventional LED, the concentration of holes differs from well to well, and it increases the closer the well is to the p-side. To work towards the development of an LED that combines uniform hole distribution with efficient hole injection and effective hole transport in the active region, we fabricated devices emitting at three different wavelengths, to provide an experimental evaluation of the hole distribution within the active region. Devices were also produced with different indium contents in the p-type InGaN layers, because this changes the height of the potential barrier and enables a study of the hole ‘reservoir’ effect.
Increasing the hole barrier by switching from p-In0.015
Ga0.985N to p-In0.035 Ga0.965 N led to an
increase in the uniformity of the intensities of three luminescence peaks, with the well located furthest from the p-type region emitting more light (see Figure 4). This stems from improved hole transport, leading to a greater uniformity of this carrier within the active region.
A valuable discussion of hole dynamics has to distinguish between hole injection efficiency and effective hole transport in the active region. Hole injection efficiency is governed by the total number of holes injected into the active region – it does not depend on which well captures them – while hole transport gives an insight into the
Further reading J. Kim et. al. IEEE Photon. Technol. Lett. 25 1789 (2013) S. Choi et. al. Appl. Phys. Lett. 101 161110 (2012) J.-H. Ryou and R. D. Dupuis Opt. Express 19 A897 (2011) S. Choi et. al. Appl. Phys. Lett. 96 221105 (2010) J.-H. Ryou et. al. IEEE J. Sel. Top. Quant. Electron. 15 1080 (2009) J. P. Liu et. al. Appl. Phys. Lett. 93 021102 (2008)
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www.compoundsemiconductor.net October 2013
Implementing those requirements is far from trivial. It may require increasing the number of wells in the LED and increasing the confinement of electrons in the active region. In addition, holes will have to be injected efficiency into the active region and transported across it very efficiently, so that this charge carrier has a fairly uniform population across the multiple-quantum-well region, even if it contains many wells.
Performing further fundamental studies and engineering of LED structures will help to uncover a route to such efficiency-droop-mitigating devices and spur the solid-state lighting revolution.
£ The authors wish to thank Jeomoh Kim, Suk Choi, Hee Jin Kim, Mi-Hee Ji and Md. M. Satter from Georgia Institute of Technology, Yong Suk Cho from the University of Houston, and Alec M. Fischer from Arizona State University for their contributions to this study of LED droop.
© 2013 Angel Business Communications. Permission required.
It is also possible that the potential barrier can limit hole injection efficiency, especially under low injection conditions. But this effect will diminish as the current is cranked up, and is expected to become negligible under high injection conditions. In that regime, injection efficiency is not strongly influenced by barrier height, but if the holes have overcome a higher barrier potential, they can be transported farther, leading to a more uniform hole distribution within the active region.
Our studies show that it is not essential to produce an unequivocal explanation for droop – which will hopefully come soon – to mitigate droop in LEDs and ultimately increase sales of light bulbs based on this technology. What we do show is that it is possible to combat droop by: reducing the carrier concentration in each well, so that Auger recombination does not kick-in at high injection conditions; making the carrier concentration in every well high enough to maximise the radiative recombination rate, while maintaining negligible Auger recombination; ensuring that in every well, the concentrations of electrons and holes are ideally identical; and trying to enable a uniform distribution of electrons and holes within the multiple quantum well.
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